Inductive conductivity sensors serve in a large number of applications in laboratory and process measurements technology for registering the specific electrical conductivity of a liquid medium. They are used preferably where large measuring ranges and high chemical or thermal loadings occur. This is the case, for example, in a large number of industrial, chemical processes, however, also in the case of hot steam sterilization methods, which are frequently applied due to hygienic requirements in the field of foods technology.
An inductive conductivity sensor includes a transmitting coil and a receiving coil, which are, as a rule, embodied as ring coils, also referred to as toroidal coils. A conductivity sensor of such type functions as a kind of double transformer, wherein the transmitting and receiving coils are inserted so far into the medium that a closed electrical current path can form extending through the medium and passing through the transmitting and receiving coils. When the transmitting coil is excited with an alternating voltage signal as an input signal, it produces a magnetic field, which induces in the closed path through the medium passing through the coils an electrical current, whose strength depends on the electrical conductivity of the medium. Since this electrical alternating electrical current in the medium, in turn, brings about a variable magnetic field surrounding it, an alternating electrical current is induced in the receiving coil as an output signal. This alternating electrical current, or a corresponding alternating voltage, delivered from the receiving coil as an output signal is a measure for the electrical conductivity of the medium.
For feeding the transmitting coil with an alternating voltage, an inductive conductivity sensor includes a driver circuit connected with the transmitting coil. For registering the output signal of the receiving coil, the conductivity sensor includes, electrically connected with the receiving coil, a receiving circuit, which is embodied to output the registered measurement signal, in given cases, conditioned by the receiving circuit, to a sensor electronics, which serves further to process the measurement signal and, in given cases, to digitize it. Frequently, conductivity sensors are embodied as measuring probes at least sectionally immersible in the medium. Such a measuring probe includes a housing, in which are accommodated the transmitting and receiving coils, in given cases, the driver circuit and the receiving circuit, as well as other circuit parts assembled with the transmitting and receiving circuits into a sensor circuit. The measuring probe is in such an embodiment connected with a separately situated superordinated unit, for example, a display unit, a measurement transmitter, a computer or a control system. The superordinated unit can be embodied both for supplying energy to the measuring probe as well as also for data communication with the measuring probe. The sensor circuit optionally contained in the measuring probe can be embodied to forward the further processed, in given cases, digitized, measurement signal to the superordinated unit. The corresponding measured value can be displayed via the superordinated unit by means of a display system or output via a data interface.
Inductive conductivity sensors have a lower limit of detection in the region of, for instance, 10-200 μS/cm. Because of this, these sensors are not applicable in the pure and cleanest water fields.
The cause for the lower measuring range limit in the case of inductive conductivity sensors is that the measurement current induced in the analyte in the case of low conductivities is very small and the existing capacitive coupling of the transmitting coil to the receiving coil produces a base signal, which is greater than or equal to the measurement signal. In other words, the signal/noise ratio is not good.
Besides the above described, inductive conductivity sensors, there are also conductive conductivity sensors. In such case, determining the conductivity in the medium occurs with a measuring arrangement, in the case of which, as in the case of a capacitor, two electrodes are located opposite one another. The electrical resistance, or its reciprocal value, the conductance, is measured using Ohm's law. The specific conductivity is ascertained from the conductance using the cell constants determined from the sensor geometry.
In the case of large conductivities, conductive conductivity sensors exhibit polarization phenomena on the electrodes, which contact the analyte. Because of the principles involved, these effects do not occur in the case of inductive conductivity sensors. Therefore, inductive conductivity sensors are applied for high conductivities and conductive conductivity sensors for low conductivities. Today, processes, in which measurements must be made both in the case of very high as well as also very low conductivities, are equipped either with two conductivity sensors, namely an inductive conductivity sensor and a conductive conductivity sensor, or with conductive, four pole sensors. The latter, however, still have a lower upper measuring limit than an inductive conductivity sensor.